1. Field of the Invention
The present invention generally relates to a composite structure and a method of manufacturing the same, and more specifically to a composite structure combining graphene platelets and carbon nanotubes with an axial direction of each carbon nanotube perpendicular to a planar direction of a planar surface of the corresponding graphene platelet to effectively separate and prevent the graphene platelets from congregating together with a three dimensional spacer formed of the carbon nanotubes for enhancing excellent performance of the graphene platelets to the actual applications, and a method of manufacturing the same.
2. The Prior Arts
Since Geim and Konstantin Novoselov successfully developed the process of tape exfoliation to obtain mono layer graphite (or called graphene) at University Manchester in 2004 and won the 2010 Nobel Prize for physics, the related industries have continuously tried to apply the excellent performance of graphene to various fields. Specifically, graphene is substantially formed of a mono layer of carbon atoms, which are tightly bonded with sp2 hybrid orbital in a two-dimensional form of hexagonal honeycomb crystal. The chemical bond for graphene is a graphitic bond, which is a hybrid bond of covalent and metal bonds. Thus, graphene inherently exhibits excellent thermal conductivity and electrical conductivity. For instance, the electronic mobility of graphene is even higher than that of carbon nanotube and silicon crystal at room temperature. Additionally, graphene has lower resistivity than copper and silver.
Furthermore, graphene has a thickness only one carbon diameter about 0.335 nm, its mechanical strength is higher than that of steel by hundreds of times, and its density is only one fourth of steel such that graphene is the thinnest and hardest material in the world. In particular, graphene possesses thermal conductivity higher than carbon nanotube and adamant, even theoretically up to 5300 W/mK, and is thus an excellent material for heat conduction and thermal dissipation.
Traditionally, the process of forming graphene generally comprises three primary types, including defoliation, direct growth and carbon nanotube conversion. The process of defoliation is more practical for mass production because graphene obtained is in a form of powder. One of the key aspects of the defoliation process is the chemical reaction including oxidization and reduction. A graphitic material is first oxidized to form graphite oxide, and separation and reduction are performed to obtain grapheme material.
Therefore, it is necessary for various applications to make graphene more controllable by chemically or physically modifying graphene. For instance, three dimensional spacers are used to modify graphene such that individual graphene platelets are separated and can be easily dispersed, instead of easily congregating or stacking up together.
In addition, graphene and carbon nanotube are advantageous in the field of transparent electrodes because of high flexibility and low reflectivity, and also one of good options for flexible electronic material. Further, graphene exhibits excellent thermal performance, and has drawn attention of many researchers for improving heat conduction and thermal dissipation. If the graphene platelets can be well dispersed to manufacture high quality thermal elements, the conduction and dissipation efficiency for the whole system can be improved and electrical performance of individual elements is greatly enhanced. However, it is more difficult to prepare a good dispersing solution for graphene platelets than the carbon nanotubes because the graphene platelets are quite different from ordinary powder and have higher specific surface area than the carbon nanotubes. The graphene platelets easily congregate together in the mixing process, thereby limiting the actual application field. Therefore, one technical problem for practically applying graphene is that a thin film formed of a mono layer of graphene with high homogeneousness is hard to obtain. If the graphene platelets are kept in contact and electrically connected together, congregation or inhomogeneous stacking is almost inevitable.
In the prior arts, some three dimensional spacers are formed to solve the above issue by blending a specific material having a different space dimension to separate the individual graphene platelets such as spherical nanoparticles, carbon nanotubes or metal nanothreads. After the spacers and the graphene powder are well mixed, congregation is effectively suppressed. For the applications requiring high electrical conductivity, the metal nanoparticles as good electrical conductors are suitable for the spacers. However, the prior arts still encounter many constraints in other applications.
U.S. Pat. No. 8,315,039 B2 disclosed graphene platelets modified for a super capacitor. Specifically, the modified graphene platelets are formed by combining metal salt and graphene platelets. The metal salt is preferably metal oxide, carbide or nitride. In addition, the surface of the graphene platelet further forms a bump shape served as the spacer such that the specific surface area of the modified graphene platelet is up to 500-1800 m2/g. The modified graphene platelets can be further functionalized to manufacture the super capacitor with capacitance up to 298 F/g. While such kind of modified graphene platelets possesses considerably high capacitance, the metal salt substantially affects or interferes with physical contact of the graphene platelets. As a result, electrical and thermal performance is greatly and adversely affected.
In another US patent, 20140030590 A1, graphene electrodes for energy storage are mentioned. The spacers and the graphene platelets are mixed without any liquid solvent to improve tap density and increase energy capacity. In consideration of crosslinking for actual applications, the spacers are selected from resin, rubber or other elastomers, and have a diameter less than 1 μm. Particularly, the mixing ratio is between 0.2-20%. The spacers effectively prevent the graphene platelets from stacking or congregating, and also help the resin uniformly cover the surface of the graphene platelets during the process of thermal auxiliary solidification for manufacturing the electrodes. However, the spacers in a form of particles are not well dispersed with the graphene platelets in the solid phase mixture. As a result, the gap among the graphene platelets stacking together is narrow, thereby greatly limiting the allowable particle size of the spacers.
Therefore, it greatly needs a composite structure of graphene and carbon nanotube and a method of manufacturing the same, wherein the carbon nanotubes are vertically formed on the planar surfaces of the graphene platelets so as to effectively separate the graphene platelets and particularly implement a network structure possessing excellent properties such as thermal conductivity and electrical conductivity, thereby overcoming the problems in the prior arts.